This invention relates to an apparatus for reproducing digital speech data optically recorded on a motion picture or the like.
If, in a transmission optical system, such as a microscope, the illumination efficiency is to be taken into account, it is desirable to equate the numerical aperture NA of the reproducing system with the numerical aperture of the illuminating system. In addition, for maximizing the resolution, it is preferred to set the numerical aperture of the illuminating system to 1.5 times as large as that of the reproducing light. Consequently, in the conventional transmitting optical system, such as the microscope, the numerical aperture of the illuminating system is set so as to not more than 1.5 times as large as that of the reproducing light. The numerical aperture of the illuminating system is given as a sine of a divergence angle of a light ray with respect to the optical axis with a pinhole placed on an illuminated surface.
It is now assumed that speech data is read from a conventional motion picture film on which speech data has been recorded as a digital pattern, and signals are read from the side of a base surface 2 of a motion picture film 1, as shown in FIG. 1. In such case, should there exist a scratch 5 on the base surface 2, and should the numerical aperture of the illuminating system be larger than the numerical aperture of the reproducing lens 4, the scratch tends to be defocused to a lesser extent, so that the light of the image of the scratch 8 on the base surface 2 is collected by the reproducing lens 4. Consequently, when the light beam collected by the reproducing lens 4 is converted into electrical signals (digital signals), the resulting signals are corrupted significantly by the scratch 5.
On the other hand, if the numerical aperture of the illuminating system is set to not more than 1.5 times of the numerical aperture of the reproducing system, the illuminating optical system of a complex structure is required, because a light guide having a larger divergence angle cannot be employed.
In addition, if the digital speech data recorded on the motion picture film i is to be reproduced, with the numerical aperture of the illuminating system being set so as to be not more than 1.5 times as large as that of the reproducing system, the scratch or dust is raised in modulation factor, so that the reproduced digital signals are affected in an increasing amount. Furthermore, if the speech data on the motion picture film 1 is to be reproduced with a higher illumination efficiency and a high S/N ratio, it is necessary to employ a reproducing lens 4 having a higher value of the numerical aperture of the reproducing system. However, in such case, the depth of focus of the reproducing lens 4 becomes shallow, such that, when the speech data on the motion picture film 1 is reproduced, the reproduced digital signals are subjected to defocusing. Conversely, when the reproducing lens 4 of an extremely deep depth of focus is employed, focusing occurs not only on an emulsifier surface having the pattern of the speech data on the motion picture film recorded thereon, but also on the base surface 2, so that the light transmitted through the scratch on the base surface 2 or the dust is also collected, and hence the reproduced digital signal is affected significantly by the scratch or dust.